In the lung, reduced glutathione (GSH) is present in high concentrations in the epithelial lining fluid (ELF) of the lower respiratory tract, with normal levels in human ELF being more than 40-fold greater than that in plasma. As such, ELF GSH is a major component of the antioxidant screen that protects the pulmonary epithelium from oxidants released by inflammatory cells as well as inhaled oxidants. In addition, ELF GSH helps maintain the normal function of the immune components of the pulmonary epithelial host defense system. However, in certain conditions, such as (1) cystic fibrosis (CF), (2) acquired immunodeficiency syndrome (AIDS), and (3) idiopathic pulmonary fibrosis (IPF), there is found to be a substantial ELF GSH deficiency.